1. Field of the Invention
The present invention relates to an image forming apparatus using an intermediate transfer body and more particularly, to an image forming apparatus in which a quality of a recorded image is improved by suppressing changes in speed of an intermediate transfer body and a photosensitive body.
2. Related Prior Art
Conventionally, in an image forming apparatus of an electrophotographic type using an intermediate transfer body in the form of a drum or a belt, a recorded image has been obtained in such a manner that an electrostatic latent image is formed by exposing a photosensitive body in the form of a drum or a belt to light (including laser beam) corresponding to image information, the electrostatic latent image is developed with toner in a developing unit, thereafter a toner image is once transferred to the intermediate transfer body, the toner image on the intermediate transfer body is further transferred to a recording paper and finally the image thus transferred is fixed on the recording paper in a fixing unit. There has been known a so-called transfix type image forming apparatus in which transfer and fixing of a toner image on an intermediate transfer body to a recording paper are simultaneously conducted.
In the mean time, in such an image forming apparatus, a recording paper has been passed between an intermediate transfer body and a transfer roller or a transfer/fixing roller to transfer a toner image on the intermediate transfer body onto a recording paper in order to realize a high speed operation when an electrostatic latent image is formed on a photosensitive body or the toner image, developed using toner, on the photosensitive body is transferred to the intermediate transfer body. Therefore, though it is preferred that no changes in speed of the intermediate transfer body and photosensitive body occur in order to obtain a good image, there arise impulsive changes in speed of the intermediate transfer body and the photosensitive body since impulsive changes in torque load on the intermediate transfer body and the photosensitive body occur due to various causes later described.
For example, when a leading edge or the following edge of a recording paper passes between an intermediate transfer body and a transfer roller or a transfer/fixing roller, a impulsive change arises in torque load of the intermediate transfer body.
In the case where a distance between a transfer unit and a fixing unit is shorter than a length of a recording paper, a shock which arises when a leading edge of the recording paper moves into the fixing unit is transmitted to an intermediate transfer body through the recording paper to cause an impulsive change in torque load of the intermediate transfer body.
In addition, when a cleaning blade, a cleaning roller or the like, which is used to remove toner on the intermediate transfer body or the photosensitive body, gets contacted with and rubbed or moved away from the surface of the intermediate transfer body or the photosensitive body, an impulsive change in torque load affects the intermediate transfer body or the photosensitive body.
Furthermore, a change in speed of a motor which drives the intermediate transfer body or the photosensitive body, and gears out of mesh with which a driving force of a motor is transmitted to the intermediate transfer body or the photosensitive body or the like, cause constant speed changes in the bodies.
As described above, if an impulsive change in torque load of the intermediate transfer body or the photosensitive body occurs, degradation in image quality arises as described below. For example, when a recording paper moves into or move out of a space between an intermediate transfer body and a transfer roller in a process of forming an electrostatic latent image on a photosensitive body, a impulsive change in torque load of the intermediate transfer member is brought about, which in turn results in an impulsive change in speed of the intermediate transfer body, which then causes an impulsive change in speed of the photosensitive body as well. As a result, a disturbance in a latent image on the photosensitive body arises, which causes the latent image affected by the disturbance to suffer from a density unevenness in a toner image produced by toner-developing. In a so-called color image forming apparatus of a tandem type, in which a plurality of photosensitive bodies for respectively forming toner images in various colors are provided along an intermediate transfer body and a toner image in each color is sequentially transferred on the intermediate transfer body, the toner images in different colors have poor overlaying alignment in forming a color image (hereinafter referred to as color misalignment) In addition to this, in the case where a distance between a transfer unit and a fixing unit is shorter than a length of a recording paper, a shock which arises when a leading edge of the recording paper moves into the fixing unit is transmitted to an intermediate transfer body through the recording paper to cause a change in speed of the intermediate transfer body, so that a smear is produced on a toner image on the recording paper.
As conventional image forming apparatuses to suppress a change in speed of an intermediate transfer body and the photosensitive body which are causes for density unevenness, color misalignment or a smear, there are disclosed such apparatuses in, for example, Japanese Published Unexamined Patent Application Nos. Hei 4-221974 and Hei 7-239589.
An image forming apparatus disclosed in Japanese Published Unexamined Patent Application No. Hei 4-221974 comprises a fly-wheel mounted at one end of a drive roller for driving a belt-like intermediate transfer body in order to increase an inertia associated with the body and thereby a change in speed of the intermediate transfer body does not occur.
An image forming apparatus disclosed in Japanese Published Unexamined Patent Application No. Hei 4-239589 comprises a frictional member contacting with and rubbing a photosensitive body drum and thereby a torque load is constantly imposed on the photosensitive body drum.
However, according to the conventional image forming apparatus disclosed in Japanese Published Unexamined Patent Application No. Hei 4-221974, a large size fly-wheel is required in order to prevent a large impulsive change in speed of the intermediate transfer body which occurs when a recording paper moves into or moves out of a space between the intermediate transfer body and a transfer roller and thus there arise problems such as increase in an apparatus's size, increase in power consumption, and the like.
In addition, according to the conventional image forming apparatus disclosed in Japanese Published Unexamined Patent Application No. Hei 7-239589, since, if the invention was applied to a color image forming apparatus of a tandem type the frictional member, a frictional member would be employed for contact and friction in each of photosensitive body drums for various colors, there would arise a structural problem that the image forming apparatus would come to have a complex structure. A change in speed of the intermediate transfer body which occurs when a recording paper moves into or moves out of a space between the intermediate transfer body and a transfer roller cannot sufficiently be prevented since the frictional member installed on the photosensitive body drum is an indirect countermeasure.